248 
Printing time.of.an orthophoto. may.vary from 10. minutes 
(flat terrain) to 23 hours (mountainous terrain). The 
typical time for relative and absolute orientation is about 
5,- Z7 minutes... Photographs taken by .cameras ‘of .55 .nm to 
315 mm focal length can be processed on the instrument. 
The orthophotos are produced at the scale identical or 
close to the scale of original photographs. 
The image quality of orthophotos is excellent, 
and also their geometry - at least in open areas - is very 
good. 
4, Stereo-orthophotos: a new concept in mapping and 
  
photointerpretation techniques 
  
So far we described briefly various systems for 
producing orthophotos. The common feature and limitation 
of the usual orthophotos is that the indispensible plot- 
ting of some of the photo map content in the usual line 
form (symbolization) often including precise contouring, 
must be done in a conventional way, that is on a conven- 
tional stereo-plotter. Also the huge volume of photo- 
interpretative work connected with all kinds of land and 
resource inventories which are becoming standard in 
national mapping, must be performed either with very 
primitive and inefficient instruments (stereoscopes, paper 
prints of photographs) or with complex photogrammetric 
instruments (proper photogrammetric stereoplotters), too 
expensive and too complex to be used by non-mappers. 
Whichever way is chosen, the process is inefficient, and 
occasionally technically incorrect. For instance, if 
the orthophoto should contain some local geometric im- 
perfections, plotting of details in their correct coordi- 
nate position on the orthophotos could be confusing and 
useless: it would be awkward to offset a property bound- 
ary by a few tenths of a millimeter in order to obtain a 
more accurate relationship between the boundary and the 
coordinate system instead of marking it along a visible 
fence where it does in fact belong. The same applies to 
contour lines and other features. 
To remedy this situation the author [ 3,4] develop- 
ed* at the National Research Council of Canada a mapping 
system based on stereo-orthophotos. In this system, any 
mapping and related operations are carried out on a three- 
dimensional, metrically correct model of the terrain form- 
ed by an orthophoto and its stereomate. The stereomate 
is an orthophoto of the second photograph of the stereo- 
pair, modified by horizontal parallaxes px, which are pro- 
portional to the elevation differences h of the terrain 
over a reference plane. That is 
Px e c.h 
  
*significant contribution was made by S.H. Collins who 
joined the author in his effort. 
  
  
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